Multiple access schemes are employed by modern radio systems to allow multiple users to share a limited amount of bandwidth, while maintaining acceptable system performance. Common multiple access schemes include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). System performance is also aided by error control codes. Nearly all communications systems rely on some form of error control for managing errors that may occur due to noise and other factors during transmission of information through a communication channel. These communications systems can include satellite systems, fiber-optic systems, cellular systems, and radio and television broadcasting systems. Efficient error control schemes implemented at the transmitting end of these communications systems have the capacity to enable the transmission of data including audio, video, text, etc., with very low error rates within a given signal-to-noise ratio (SNR) environment. Powerful error control schemes also enable a communication system to achieve target error performance rates in environments with very low SNR, such as in satellite and other wireless systems where noise is prevalent and high levels of transmission power are costly, if even feasible.
Interleave Division Multiple Access (IDMA) is a multiple access technique where different users that share the same bandwidth and time slots are separated by user specific interleavers. As the bandwidth and power become scarce to support the ever increasing throughput requirements, more complex but more efficient techniques play more important roles in future communication systems. IDMA is an effective technique that trades extra receiver complexity with bandwidth and power savings. On the other hand, in systems where the number of users is high and the block size is large, storage of a high number of long interleavers may be undesirable. Scrambled Coded Multiple Access (SCMA) addresses this complexity by using a single scrambling sequence with different shift factors for different users without any performance penalty. With SCMA, the user specific interleavers of IDMA are replaced with user specific scrambler sequences. While there is no noticeable performance difference between the two approaches, generation and implementation of scrambler sequences is significantly simpler. In fact, the same scrambler sequence with different rotation factors can be used for different users with no impact on performance, which further reduces receiver complexity. With SCMA, therefore, all of the benefits of IDMA are achieved with reduced complexity.
Similar to IDMA or random waveform Code Division Multiple Access (CDMA), SCMA is a non-orthogonal multiple access technique. While orthogonal multiple access schemes such as Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA) are implicitly too restrictive to achieve theoretical limits in fading channels, non-orthogonal CDMA, IDMA or SCMA have the potential of achieving these limits. Further, as discussed above FEC coding is typically used to improve the performance. The main difference between CDMA and SCMA is that, while in CDMA different users are separated with different signature sequences with a spreading factor greater than one, in SCMA even a spreading factor of one would be enough to detect overlapped users based on user specific scrambler sequences and iterative multiuser cancellation with FEC decoding. As a result, the available bandwidth can be used for very low rate coding which gives SCMA extra coding gain that is not available in CDMA. Actually it is also possible to use SCMA with a spreading factor greater than one. Another benefit of the iterative receiver structure of SCMA is that the system performance actually improves with power variations among the users, which eliminates the need of power control, an important requirement of traditional CDMA.
Further, in conventional burst mode communication systems, a transmitter transmits burst mode signals at a certain frequency, phase and timing, which is received by a receiver through a communication channel. In conventional burst mode communication systems, it is necessary to quickly estimate various parameters of the received bursts as they arrive. These parameters include detection of the presence of a burst (start time), frequency, initial phase, timing and amplitude. In typical burst transmission systems, a unique word is used to facilitate the identification of the beginning of a transmitted burst and the determination of phase offset, by the receiver. The term “Unique Word” (UW) refers to a known, pre-determined pattern (known a priori to the receiver) that is transmitted at the beginning of each burst, whereby the receiver detects the UW and synchronizes with the received bursts (i.e., the receiver estimates the burst parameters based on the detected UW).
At the receiver, iterative multiuser detection or interference cancellation followed by decoding is performed to approach maximum likelihood (ML) receiver performance without excessive complexity. But for coded CDMA systems, even this iterative receiver may lead to complicated algorithms especially when the number of users is large. Typically with CDMA, the complexity of multiuser detection or soft interference cancellation algorithms grows in polynomial form with the number of users/user terminals. On the other hand, similar to IDMA, SCMA lends itself to a simple chip by chip detection algorithm whose total complexity grows only linearly with the number of users. Further, uncoded SCMA systems perform at least as well as and usually better than uncoded CDMA, and the performance gap between the two classes of schemes grows bigger for heavily loaded systems.
Moreover (as discussed in further detail below), in a large-scale shared bandwidth communications system, a number of channels may be designated as random access contention channels for terminals to send short messages. In a TDMA system, such random access contention channels are typically implemented as slotted Aloha (S-Aloha) channels. The terminal in a TDMA based network may use the S-Aloha contention channel for send control messages to a respective gateway to request a dedicated channel for a communications session (e.g., a web browsing session). The terminal then waits for assignment of the dedicated channel before beginning the session data transmissions. With much higher efficiency, however, SCMA can be employed for a contention channel to send short messages directly, without requesting and waiting for assignment of dedicated bandwidth. Moreover, such applications for an SCMA channel are becoming increasingly important for delay sensitive communications that cannot afford the added latency of the dedicated bandwidth request and bandwidth assignment response. Further, a reduction of latency has become more important for web browsing applications for faster response times and improved user experience.
Further, with respect to web browsing sessions, each webpage request transmitted by the client terminal or web browser typically requires more data to be transmitted, as webpages have become more complex (e.g., on average, a typical webpage contains upwards of 30-60 objects). A typical webpage request would require many SCMA bursts to complete.
While the complexity of SCMA grows only linearly with the number of users, however, with larger systems (e.g., having upwards of tens or hundreds of thousands of user terminals), SCMA system implementations can become relatively complex with each user/user terminal having a distinct scrambling signature. Further, with regard to applications that may require several SCMA bursts for a message (e.g., a webpage request message), the receiver complexity compounds the delay in processing such a request. What is needed, therefore, is an approach for an SCMA protocol, where applications may require several SCMA bursts for a message, that facilitates more efficient and streamlined processing at the receiver, and consequently reduces the processing time.